So... Does this mean physical stamp buyers need to do anything to ensure it's not redeemed/whatever? I guess I'm more confused now..... How do you know the eth addy unless you bought via OCS? Is there any way to tell if a physical has been redeemed other than scratching? My point is - could someone sell a physical stamp as "unredeemed", when in fact it is not? Thanks.... 
Maybe this can help as well, just my way of explaining what these stamps are...
Using the Crypto Stamp:
Each stamp has 2 parts.
The left part, which is stylized with a unicorn associated with Ethereum,
functions as a "standard stamp" that can be used to send mail. (€6,90)
This part has a QR code of the digital asset/Token
The right part, on the other hand, contains the credentials used to
authenticate the crypto collectible (digital asset/Token) via blockchain.
This is the “digital stamp” and contains 3 codes [1], [2] & [3]
This "digital stamp" (right part) is like a vault with a inside a wallet
which contains a precious gem (= digital asset/Token).
The wallet contains the digital asset/Token CS1 (CryptoStamp 1)
and Ξ 0.001666666666666666 (= ETH or ETHEREUM, a crypto currency)
which can be use to pay for transactions/transfers.
Code [1] is, so of speak, the serial number of the gem (digital asset/Token)
= 1 of 150.000 pieces, each one has 1 (digital) colour
(Black, Green, Blue, Yellow or Red).
When seeing the physical stamp, it isn't possible to see its (digital) colour.
Black, Green, Blue, Yellow or Red, physically they all have the same appearance.
Scanning the QR code or entering the "code [1]" online at "crypto.post.at/CS1" (> "asset")
reveals the digital colour of the digital asset/Token (Black, Green, Blue, Yellow or Red),
as well as its history! (for example which ETH address purchased it, and when)
Anyone can see what the colour is of each Token,
but only the owner can do something with the Token.
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Using the "digital asset/Token":
The 150.000 are/were sold through 1 series of 149.500 pieces and another series of 500 pieces.
Series 1:
When you have purchased 1 of the 149.500 stamps which has a “paper wallet”
you can sell this "vault" (right part, containing the wallet and gem locked inside)
together with the left part (€6,90) to anyone. Only whoever holds the key can unlock the vault,
and then sell the "gem" (digital asset/Token). The gem goes then from the wallet to another one’s wallet.
Everything is recorded on the Ethereum Blockchain.
The key to open the vault is scratching the scratch fields.
The code hidden under scratch field [2] is the (address of the) wallet.
The code hidden under scratch field [3] is the key to open and use the wallet
which makes it possible to sell and/or transfer the digital asset/Token.
Then you’ll have
1 scratched physical stamp and
1 saleable/transferable digital asset/Token
Series 2:
When you have purchased 1 of the 500 stamps
the "gem" (digital asset/Token) isn’t in its originally assigned "vault",
the Token is already out of its ("first") wallet
and is put digitally into the new owner’s wallet.
The "digital asset/Token" can already be sold separately.
Furthermore, the key to unlock the "vault" is unused as
the digital asset/Token wasn't in the 1st wallet to begin with.
The "vault" is kept closed because the keys (under the scratch fields) have
no use anymore, therefore the scratch fields on the physical stamp remain intact!
In this case you’ll have
1 unscratched physical stamp and
1 saleable/transferable digital asset/Token
which both can be sold separately.
Hope this helps...
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